Aircraft control surfaces, such as ailerons, rudders, and elevators, are typically mounted on a fixed airfoil and are pivotable from a neutral position in opposite directions. The sum of the pivotal deflections from the neutral position is commonly 60.degree. and may be even higher in certain special applications. In order to maximize the control force on the aircraft created by the deflection of the control surface, it is necessary to avoid flow separation on the suction side of the control surface. One factor that can cause separation is discontinuities in the transition contour between the fixed airfoil and the deflected control surface. Therefore, it is desirable to maintain a smooth and continuous transition contour on at least the suction side regardless of the position of the control surface. Another condition that can cause separation is cross flow from the pressure side to the suction side. Sealing along the airfoil/control surface interface on at least one side of the control surface is, thus, generally required for all control surface positions.
FIG. 1 illustrates a common approach to the design of an airfoil/control surface interface that maintains a relatively smooth and continuous transition on the suction side and avoids cross flow for all control surface positions. The illustrated interface is between an aircraft wing 2 and a trailing edge aileron 4. The aileron 4 is mounted to pivot about a pivot axis X both upwardly and downwardly to a maximum deflection angle of .delta.. As shown, the deflection angle .delta. is 30.degree.. Each of the upper and lower leading edges of the aileron 4 carries a curved fairing 6, 8. The upper and lower trailing edges of the wing 2 each have a fixed trailing edge panel that carries a seal 10 which seals against the adjacent fairing 6, 8. Each of the fairings 6, 8 overlaps its adjacent panel, and more specifically the seal 10 carried by the panel, by an angular distance equal to the angle of deflection .delta.. At least this much overlap is required to provide complete sealing throughout the deflection range of the aileron 4.
FIG. 1 shows the neutral position of the fairings 6, 8, when the aileron 4 is undeflected, in solid lines and the deflected positions in broken lines. This illustrates the requirement that there be room inside the wing structure for each of the fairings 6, 8 to move an angular distance of 2.delta. beyond its seal 10. When this requirement is met, the only remaining available space AS for hinge and actuation structure is extremely limited. The space AS is insufficient to accommodate known types of structure for hinge fittings and actuation.
FIG. 2 illustrates a common practice that is employed to overcome the problem of insufficient space illustrated in FIG. 1. This practice is to trim the leading edge fairings 6, 8 at key locations to reduce or eliminate the overlap between the fairings and the wing's fixed trailing edge. FIG. 2 shows an aileron 14 pivotally mounted on the trailing edge of a wing 12. The trimmed upper and lower fairings are represented by the reference numerals 16, 18, respectively. As in FIG. 1, each of the corresponding trailing edges of the wing 12 has a fixed panel that carries a seal 10. FIG. 2 schematically represents a hinge location at which the overlap has been eliminated to provide space for hinge fittings 22, 26 mounted on spars 20, 24 that are a part of the structure of the wing 12 and the aileron 14, respectively. Because of the elimination of the overlap, the space beyond the seals 10 required for the fairings 16, 18 is .delta., rather than 2.delta., as in FIG. 1. FIG. 3 shows a typical trim pattern on the lower leading edge fairing 18 of an aileron. As shown in FIG. 3, the trim locations include five hinge fitting locations 28 and two actuator locations 30.
While the approach illustrated in FIGS. 2 and 3 solves the problem of inadequate space, it presents another problem. When the control surface 14 is deflected downwardly, a gap G opens up on the upper suction side of the control surface 14. Even though the trimming is done only locally, where required for hinges and actuators, the gaps on the suction side can cause substantial downstream flow separation which diminishes the effectiveness of the control surface and produces drag.
The problems discussed above are illustrated and described in terms of a wing trailing edge aileron. The same problems also apply to other types of control surfaces, including leading edge control surfaces, both leading edge and trailing edge control surfaces on structures other than wings, such as elevators, and vertical control surfaces, such as rudders. In the last-mentioned case, deflection would be in opposite lateral directions rather than up and down.